As is known, nuclear reactors consist of an array of fuel rods containing the nuclear fuel. The fuel rods are metal tubes, typically from 8 to 15 feet in length and about 1/2 inch in diameter, and are supported in groups of fuel assemblies which may comprise a considerable number of rods. The large reactors utilized for power generation contain a large number of these fuel assemblies arranged in a suitable configuration.
After an extended period of operation, the irradiated or spent fuel assemblies must be removed from the reactor and replaced. The spent fuel rods contain residual amounts of the original fuel material, and various amount of numerous fission products resulting from fission of the nuclei of the original fuel. Other nuclear reactions within the reactor transmute some of the elements present into new materials. Certain of these materials are themselves fissionable. Many of the fission products and new elements are highly radioactive, at least initially, and thus produce considerable heat and the entire fuel assembly is dangerously radioactive. The fuel rods can be reprocessed by chemically separating the fissionable material for reuse as fuel and recovering the various other fission products, or the fuel assemblies may be disposed of by placing them in permanent storage, or otherwise.
When radioactive fuel elements of the type described above are discharged from a nuclear reactor, they are normally stored, at least initially, in a water-filled pool which serves two purposes. The first of these purposes is to permit a cooling period to occur so that the radioactivity of the fuel assemblies may decline and they can be more easily handled. Secondly, the pool forms a convenient temporary storage place for the elements until some permanent disposal means such as reprocessing is employed.
Because of the cost of storage facilities for spent nuclear fuel assemblies, it is desirable to store as many elements as practical in the pool; and elaborate racks have been devised to permit close physical packing of these elements. The limiting factor in the number of fuel assemblies contained in a pool is the fact that the used fuel elements still contain fissionable materials as explained above; and if a given number of elements having a sufficient fissionable material content is placed together in certain configurations, the possibility exists that a self-sustaining neutron chain reaction (i.e., criticality) might be set up in the pool. It is, therefore, desirable to have available some form of instrumentation capable of monitoring the neutron multiplication in the pool to insure that criticality does not occur. As fuel assemblies are added or moved around within the pool, care must be taken that a neutron multiplication factor does not approach unity too closely. (A multiplication factor of 1 is called a critical condition and is that condition of the self-sustaining neutron chain reaction). The mutiplication factor (k) may be measured directly or the departure from criticality, loosely (k-1), which is called the reactivity, may be measured. Hence, the term shutdown reactivity meter is often employed as a generic name of such devices.
In the past, shutdown reactivity meters have been used principally in reactors that are capable of deliberately going critical. In particular, the critical point is usually used as a reference point in the calibration of the instrument. Two such devices have been prominently mentioned in the literature. The first of these consists of a neutron detector that is coupled to an analog or digital computer which solves the kinetic equations describing the physical behavior of reactors. One such device, for example, is described in an article by G. S. Stubbs entitled "Design and Use of the Reactivity Computer", IRE Transactions on Nuclear Science, March 1957. A reactivity metering system of this type can be operated in critical, subcritical (i.e., k less than unity) or supercritical (i.e., k greater than unity) modes but depends on dynamic signals from a neutron detector which can be calibrated at zero reactivity. A system of this sort is primarily used in large test or power reactors to measure control rod calibrations and shutdown reactivity. In its described form, it is not particularly suited to a potential reactor without control rods that presumably does not reach criticality.
The second type of shutdown reactivity meter operates on neutron noise signals from a detector monitoring the neutron power level in the reactor. One such meter, for example, is described in an article by M. A. Schultz, Applicant herein, entitled "Measurement of Shutdown Reactivity in Large Gamma Fields", Neutron Noise, Waves and Pulse Propagation, R. Uhrig, Ed, USAC Publication, May 1967. In this type of reactivity meter, the neutron noise signals are analyzed for frequency content; and from this information the transfer function, and hence the shutdown reactivity, can be inferred. Again, it is essential that the critical configuration transfer function be confirmed for the meter to be accurate. In addition, large efficient neutron detectors placed as closely as possible to the reactor must be used to obtain useful noise information. Furthermore, neither of the two above-described prior art systems are capable of reading multiplication factors below about 0.9.